Containers are commonly used for the packaging of food and for a wide variety of other items wherein a lid is bonded to the container e.g. by the application of heat. The containers and lids can be made of a number of materials, for example plastic, metal, or combinations thereof, whereas the lid is typically supplied in form of a continuous film and subsequently cut into a suitable shape either before or after bonding. In some examples, a plastic lid is pre-cut into a suitable shape and subsequently heat bonded onto a plastic tray.
One method of bonding the lid to the tray involves use of a laminated plastic lid having a layer of metal foil. A power supply provides an electrical current to a nearby induction coil, which induces an electrical current into the metal foil to develop heat, which melts portions of the lid and container, and fuses the lid to the container lip. For example, EP0469296 discloses an induction sealing assembly using a single turn coil to seal a plastic lid onto a plastic container. The assembly includes a nest having a recess for holding a container to be sealed, and a movable sealing head for holding a lid or foil membrane and for positioning the lid relative to an opening in the container. Means are provided to secure a portion of the sealing head against a portion of the nest to form an airtight chamber between a lower portion of the sealing head and an upper portion of the nest. The induction sealing assembly uses a vacuum source and a source of inert gas to flush air from the container prior to sealing. An induction coil mounted in the sealing head seals the lid to the container by heating some portions of the lid in contact with the container. This solution uses a precut lid, so that means for transporting and loading the lid into a proper position for sealing are required, compromising the efficiency of the packaging process. Moreover, EP0469296 generally suggests using a roll stock foil membrane to seal the container while being silent regarding the specific manner as to how the membrane should be cut from the roll in order to form an accurately fitting tray lid.
In order to package products, in particular food products, vacuum packaging has been developed and refined in the past. Among known vacuum packaging processes, vacuum skin packaging is commonly employed for packaging food products such as fresh and frozen meat and fish, cheese, processed meat, ready-to-eat meals and the like. Vacuum skin packaging is described, for example, in FR 1258357, FR 1286018, AU 3491504, US RE 30009, U.S. Pat. Nos. 3,574,642, 3,681,092, 3,713,849, 4,055,672, and 5,346,735.
Vacuum skin packaging is basically a thermoforming process. In particular, the product is typically placed on a rigid or semi-rigid support (such as a tray, a bowl, or a cup). The support with the product placed thereon is put in a vacuum chamber, where a film of thermoplastic material, held by a local vacuum in a position above the product placed on the support, is heated to make it more yielding. The space between the support and the film is evacuated. The film is then sealed at the perimeter of the tray to fix its position relative to the tray and the local vacuum above the film is shut off, releasing the film. When the vacuum inside the chamber is decreased, the vacuum inside the tray causes the film to drape down all around the product and seal to the surface of the support not covered by the product, thus forming a tight skin around the product and on the support.
US 2007/0022717 discloses a machine for gas tight packaging an object using a film material. The machine has a lower tool for supporting two trays and an upper tool having cutting devices housed inside the upper tool and facing the lower tool. A film is interposed between the upper tool and the lower tool. The upper and lower tools are first closed the one against the other and then the film is cut to the size of the peripheral rims of the trays by the cutting devices operative inside the upper tool. Sealing tools heat-seal the cut regions of the film to the peripheral rim of the tray. A vacuum is situated in the surrounding region of the tray to cause deep drawing of the film. In the document it is also mentioned that the same device can be used for sealing trays with films that are not deep drawn to form a skin over the product.
US 2005/0257501 discloses a machine for packaging a product arranged in a tray. The machine has a lower tool for supporting the tray and an upper tool with a cutting device. During operation, the film is clamped along an edge surrounding the tray and is deformed by the upper tool in a direction extending away from the product. The space surrounding the product is then evacuated, the film and the edge of the tray are sealed, and the film is subsequently cut by the cutting device.
As to the machines disclosed in US 2007/0022717 and US 2005/0257501, the film is cut to the size of the tray within the chamber formed by the upper tool and the lower tool, by means of the cutting devices provided on the upper tool. First of all, this disadvantageously requires providing a rather complex and bulky upper tool. Further, this disadvantageously requires providing an excess film with respect to the size of the support, the excess film having to be cut from the package and to be disposed of during the packaging process or at the end thereof. Indeed, the film is in the form of a continuous sheet wound on a roll (as shown e.g. in FIG. 3 of US 2005/0257501). Therefore, excess film material is required to allow the film to be pulled from the roll and to be held in place above the supported product. Further, in US 2007/0022717 more than one product loaded support (namely two) is packaged at each cycle, so that an excess film is also present between adjacent supports.
WO2011/012652 shows an apparatus for packaging a product in a tray. The machine comprises a first film transfer plate configured for holding a film sheet, heating the film sheet, bringing the film sheet to a position above a tray with the product arranged thereon, and fixing the film sheet to the tray in an air tight manner. A second film transfer plate is also present. As for the first film transfer plate also the second film transfer plate is configured for holding a film sheet, heating the film sheet, bringing the film sheet to a position above a tray with the product arranged thereon, and fixing the film sheet to the tray in an air tight manner. During a first operating step of the machine, the first film transfer plate holds a first film sheet and heats the first film sheet, while the second film transfer plate releases a second film sheet thereby allowing the second sheet to be drawn into a first tray; and during a second operating step of the machine, the second film transfer plate holds a third film sheet and heats the third film sheet, while the first film transfer plate releases the first film sheet thereby allowing the first film sheet to be drawn into a second tray. The machine further comprises a rotating cylinder suitable for rotating about its axis X, the first film transfer plate and the second film transfer plate being connected to the rotating cylinder so that, when the rotating cylinder rotates about its axis X, the positions of the first film transfer plate and the second film transfer plate are exchanged. A vacuum arrangement allows removing air from within the tray underneath the film sheet (positioned either by the first or by the second film transfer plate) through the at least one hole present in the tray. The film transfer plates are configured to release the film sheet thereby allowing the film sheet to be drawn into the tray while the vacuum arrangement is removing air from within the tray.
As can be seen from the description above, the use of pre-cut film provides an improvement over known methods and apparatus for packaging products, which rely on excess film being cut from packaged products, after packaging and sealing thereof. If the dimensions of the film—in particular the width thereof—and those of the trays are carefully adapted to one another, packaging of products using tray lidding and skin packaging can be realized effectively without excess film material having to be cut from the packaged products. This leads to substantial advantages with respect to the minimization of material consumption.
However, as shown in some examples above, excess film material can be utilized in terms of positioning the film so that requirements with respect to the accuracy of positioning the film can be much less strict. Further, in terms of handling the film, having excess portions around the packaged product allows for easy handling of the film by the excess portions being acted upon by the packaging apparatus. For example, excess portions of the film can be clamped in order to pull the film into position. In other examples, the amount of material in the excess portions can provide a stiffness to the excess portion preventing unwanted folding or other deformations and/or movements that can be detrimental to the packaging process and/or that could lead to imperfections in the packaging (e.g. compromised seal or uneven appearance).
Consequently, minimizing or eliminating the amount of excess material necessary for the packaging process can lead to one or more of the following detrimental effects.
In tray lidding, a pre-cut lid is positioned on a film holder above a tray to be sealed so that the pre-cut film extends beyond the film holder. This facilitates a sealing tool to act upon the portions of the film extending beyond the film holder in order to seal the film to the tray. However, the portions of the pre-cut film extending beyond the film holder are prone to deformation due to a number of packaging process related factors: heat emitted from the sealing tool or other components, turbulence of gas or air created upon evacuation and/or upon creation of the inert atmosphere, mechanical movement of components of the packaging tool, mechanical resistance and/or rigidity of the material at certain process related temperatures, etc. When such deformation occurs, the quality of the seal can be compromised, possibly leading to a seal of mediocre quality or a defective seal.
In skin packaging, portions of pre-cut film extending beyond the sealing tool typically extend from the corners of the tool, in particular, when the pre-cut film has a rectangular outline and pointy corners, while the tray has a curved outline and curved corners. Here, too, the film can deform and impact the quality of the seal and packaging.
Therefore, it is an object of the invention conceiving a process and an apparatus, which can efficiently and accurately position and configure a film in tray lidding or skin packaging. In particular, it is an object of the invention to position and configure peripheral portions of the film.
A further object of the invention is to provide a packaging process and apparatus that facilitate an effective control of the temperature of a film and/or peripheral portions thereof in order to prevent or eliminate undesired shrinking of the material of the film.